In recent decades, a Storage Device system based on a Hard Disk Drive (HDD) was used for a main Storage Device of Computer System; but now, some changes are observed. The reason comes from a NAND flash memory. In recent years, a Storage Device based on a NAND flash memory has been frequently used for a computer system, since the device provides advantages of high performance, low power consumption, high reliability, and small form factor, when compared to the existing HDD.
It is predicted that a market breadth of a Solid State Drive (SSD) would grow drastically in every year, since 2009. However, a market share of the NAND flash memory is small rather than HDD's. The reason comes from disadvantages of high price and equitable performance to the existing HDD, under a certain operation of an arbitrary write.
The disadvantage of high price of the NAND flash memory is being improved, by providing a high-capacity and a price decline through a Multi-level Cell (MLC). However, the SSD is still high priced compared to the HDD for a consumer. In this regard, various inventions and researches have been made in order to enlarge the market share of the storage device based on the NAND flash memory.
However, there were limits to enhance performance capabilities due to the physical aspect of the NAND flash memory (a proposal of withdrawing an erasure operation). In this regard, many companies or laboratories had conducted researches for developing a new nonvolatile RAM device to replace the NAND flash memory. As a result, a Phases change Memory (hereinafter, “PCM”) and nonvolatile RAM devices such as FRAM, MRAM had been developed and still in researching for the commercialization.
These nonvolatile RAMs are a new nonvolatile memory device which overcame the disadvantages of the existing NAND flash memory. It is expected that they will be used as a storage device to replace the storage device comprised of the existing NAND flash memory only, by being used with the NAND flash memory in next generations.
Meanwhile, it is expected that the PCM is the most influential nonvolatile RAM to replace the NAND flash memory among various nonvolatile RAMs; many companies specialized in a semiconductor are now in the process of researching for the commercialization of the PCM, with enormous capital and time investments. The reasons why the PCM is considered as a storage device to replace the NAND flash memory are described as follows:
Firstly, the PCM is not configured to have a subtraction arithmetic operation unlike the NAND flash memory. The subtraction arithmetic operation most negatively influences on the performance of the NAND flash memory; the subtraction arithmetic operation is conducted per Block unit, and to have a quite long time for the arithmetic operation, about 1 ms. Due to the subtraction arithmetic operation, it can be assumed why the NAND flash memory cannot show a higher performance than the HDD in a certain arithmetic operation of an arbitrary write. However, the PCM is not configured to have the subtraction arithmetic operation; thus, high performance capability can be obtained, compared to the NAND flash memory and the HDD.
Secondly, the PCM is configured to basically read/write per bite; the NAND flash memory is configured to read/write per page. That is, the NAND flash memory requires a whole page in order to read/write data consisting of several bites; however, the PCM may read/write only necessary data. Therefore, the PCM may show a high performance in reading/writing in comparison with the NAND flash memory.
Thirdly, the number of the read/write operations of the PCM is defensively 106, and the PCM has a lifespan that is ten times longer than the NAND flash memory. Although, in other researches for the PCM, the number of the write operation of the PCM was higher than 106; however, the number of the write operation of the test product of the PCM is approximately 106. Since the durability of the PCM is higher than that of the NAND flash memory as described above, it is expected that the PCM can replace the NAND flash memory.
However, researches for an integration degree and a commercialization of the PCM are rather insufficient than the NAND flash memory. Therefore, various researches for overcoming the disadvantages of the NAND flash memory are conducted in order for the PCM to be used with the NAND flash memory.
Lots of researches for overcoming the disadvantages of the NAND flash memory in a storage media or an operating system have been conducted, in order for the PCM to be used with the NAND flash memory; however, the researches for using the PCM and the NAND flash memory in a database system are hard to find.
The durability of the PCM has been enhanced in comparison with the NAND flash memory; however, the durability is not enough to be used as a storage device of a database. Thus, the insufficient durability issue should be solved in order for the PCM to be used as the storage device of the database.
Meanwhile, in a B+ tree which is an essential construction of material used in the database system, when analyzing a difference of the number of the write operation between each node, it can be observed that the difference of the number of the write operation between each node is quite big. Specifically, a greater number of the write operation is generated rather than other nodes, in a certain index node and a leaf node. Additionally, in the B+ tree, when a new record is inserted, the number of the write operation in a middle area of a node tends to be higher than other areas by generally using an insertion sort operation. Further, as regards a primary area of a node, every time a new node generates, the write operation is made by recording the number of records of the node, thus, the number of the write operation in one node can be the highest.
As described above, concentration of the write operation for a certain area of the B+ tree should be solved in order for the PCM to be used as a storage device of a database, because the concentration would severely harm the durability of the nonvolatile RAM such as a PCM. In order to solve the durability issue of the PCM, a method of storing the number of the write operation may be used, as the NAND flash memory performs. However, the PCM is configured to read/write per bite. So, the method cannot be used practically. In order for the PCM to be used as a storage device of the database system or the file system, a new method of securing durability is required.